The demands for smaller high frequency communications system components require smaller RF/microwave function blocks without compromising system performance. While meeting the ultimate design parameters of the system is important, the system components must also be capable of being manufactured in a cost effective manner. A variety of planar transmission structures are available for the design of miniature surface mount components. These structures include, but are not limited to, microstrip, stripline, suspended stripline and coplanar waveguides.
Transmission lines are used in radio frequency (RF) and microwave frequency circuits to guide high frequency signals between points along a particular path in an RF or microwave system. Transmission lines exist in several configurations, and for a given application, the use of a particular type of transmission line is partially based on device parameters such as the operating frequency, power level, and performance requirements. For frequencies up to 1000 MHz, parallel conductors and coaxial are typically used. For higher frequencies, stripline and microstrip techniques are typically used and upwards of 18 GHz, conventional circuit elements and transmission lines are usually replaced with cavity and waveguide techniques.
Common forms of transmission line circuitry include microstrip and stripline. A microstrip transmission line typically includes a conductor pattern on top of a dielectric substrate with a ground layer on the bottom of the substrate. One advantage of the microstrip transmission line is that it is easy to manufacture and components such as resistors, capacitors, and transistors can easily be attached to the upper surface of the microstrip to ensure that the transmission line meets operating parameters. However, one disadvantage of the microstrip transmission line is that the electrical field is not confined to the dielectric substrate and hence the modeling is based on approximation.
A typical stripline transmission line has a flat conductor disposed between two dielectric layers of material, which separate the conductor from two ground planes. The field distribution is confined to the dielectric layers between ground planes and the modeling is quite accurate. However, the typical stripline design is limited in that it is difficult to add non-planar 3-D components, such as resistors, capacitors, inductors, diodes, transistors, etc., to the stripline circuit because it is sandwiched between dielectric. It is, consequently, more difficult to adjust the operating parameters than a microstrip.
Because of the different limitations of stripline and microstrip transmission lines, typically an entire microwave system is not formed with only stripline components or only microstrip components. Typically, a microwave system uses a combination of stripline and microstrip elements connected together by coaxial cables or some other transition between them in a horizontal plane. The circuits are often placed adjacent to each other. Alternatively, multi-layer circuits may utilize stripline transmission lines stacked together with the ground planes of each transmission line separating the circuit conductors. Connection between the various stripline transmission lines may be accomplished by using plated through holes or electromagnetic coupling by forming an open slot in a ground plane separating stripline transmission lines. Formation of slots through the various layers, however, is not generally a cost effective way of establishing a connection between transmission lines.
Another way of manufacturing microwave components involves thick film techniques. For example, a thick film balanced line can be manufactured by depositing two independent metal layers with one thin dielectric layer separating them. Thick film manufacturing techniques have several drawbacks. First, thick film manufacturing processes require specialized manufacturing facilities and equipment. In addition, it is generally difficult to precisely control the dimensions and thickness of the dielectric layer and properly align the metal layers using thick film manufacturing techniques. Specifically, since the width of balanced lines is generally in the range of 0.003 to 0.005 inches, it is difficult to accurately maintain the width of a balanced line within 0.0005 inches. Such manufacturing processes also make it somewhat difficult to maintain the uniformity of a line over a given length. Since the electrical performance of a balanced line depends upon the physical dimensions of the line, the dielectric thickness that separates the conductive layers, as well as the alignment between the metal layers on either side of the dielectric layer, any type of variations in the physical dimensions of the line tend to greatly affect the performance of the balanced line.
There is a need to provide compact RF/microwave components and systems that can be manufactured using low cost fabrication processes. The processes used to manufacture such components should have the ability to provide conductors having uniform conductor line width. In addition, it would be advantageous if operating parameters of the finished component could be optimized so that the component performs according to design specifications. Desirably, the finished component should have small length and width dimensions so that the finished component occupies a small area.
One aspect of the invention relates to a multi-layer microwave circuit including first and second transmission lines arranged in a vertically stacked relationship. The first transmission line has a bottom dielectric layer and a middle dielectric layer. The dielectric layers have top and bottom surfaces. The first transmission line further includes a conductive pattern disposed between the bottom surface of middle dielectric layer and the top surface of the bottom dielectric layer. A bottom ground layer is in contact with bottom surface of the bottom dielectric layer. The second transmission line is arranged in a vertically stacked relationship above said first transmission line. The second transmission line has a top dielectric layer having top and bottom surfaces. A conductive pattern is in contact with the top surface of the top dielectric layer. According to this aspect, at least one ground layer separates the bottom surface of the top dielectric layer and the top surface of the middle dielectric layer of the first transmission line. Thus, either the ground layer and a layer overlaying the bottom surface of the top dielectric layer or a ground layer overlaying the top surface of the middle dielectric layer. Alternatively, a ground plane may overlie both the bottom surface of the top dielectric layer and the top surface of the middle dielectric layer. Electrical connection is established between the first and second transmission lines by at least one plated through hole extending through the second transmission line and contacting the conductive patterns in both the first transmission line and the second transmission line.
According to a preferred aspect of the invention the dielectric layers include a ceramic-filled soft substrate. Preferably, the ceramic-filled soft substrate has a dielectric constant of about 3 to 10 and a thickness of about 0.004 to 0.031 inches. Preferably, the ground layers include copper. The microwave circuits of the present invention can be used to manufacture a variety of microwave devices such as power splitter (also known as a power divider), phase shifter and mixers. Advantageously, the multi-layer circuit with a microstrip transmission line on the top surface of circuit provides a device that can be optimized by the addition and/or adjustment of components including, but not limited to, transistors, capacitors, and resistors. Another advantage is that multi-layer devices according to the present invention occupy less real estate than devices that include horizontally connected transmission lines. For example, a two way power splitter was fabricated according to the present invention having a length dimension of 0.3 inches and a width dimension of about 0.25 inches.
Another aspect of the invention involves a method of making a microwave component. The method includes the steps of providing first, second and third dielectric substrates, each having top and bottom surfaces. The bottom surface of the first dielectric substrate has a conductive material thereon and forms a ground plane. The top and bottom surfaces of the second and third dielectric substrates both have conductive material thereon. The method further involves removing at least a portion of the conductive material from the bottom surface of the second dielectric substrate to form a conductive pattern. Likewise, a portion of the conductive material on the top surface of the third dielectric substrate is removed to form a conductive pattern. The first, second and third dielectric substrates are arranged in a vertically stacked relationship, wherein the third substrate overlies the second substrate so that a ground plane is formed between the third and second dielectric substrates and the second substrate overlies the first substrate. The conductive pattern on the third dielectric substrate is electrically connected with the conductive pattern on the second dielectric substrate by forming a plated through hole extending through the third dielectric substrate and the second dielectric substrate and contacting the conductive patterns on the third dielectric substrate and the second dielectric substrate. Preferably, the step of forming the conductive pattern includes etching the pattern.
The invention provides a relatively simple and flexible way of providing microwave circuits and components including these circuits. Standard manufacturing techniques can be utilized to connect the various layers of the transmission lines. Optimization of the components is facilitated by having a microstrip circuit on the top surface of the multi-layer circuit structure. Additional advantages of the invention will be set forth in the following detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.